Testing of electricity meters with interconnected current and voltage circuits i.e. closed I-P links is an increasing need for meter manufacturers and meter operators. The meters that do not allow to open the links between the current and voltage measuring circuits (I-P links) for test or calibration purposes are increasingly in use. There are several reasons for this but the most important for manufacturers is the lower manufacturing cost of meters using resistive shunts for current measurement. To provide the facility to isolate the current and voltage paths of these meters would result in a significantly higher manufacturing cost. During normal operation, this isolation would not even be technically feasible. A reason for using meters with non-removable I-P links is to prevent their misuse for fraud. Another reason for testing meters with interconnected current and voltage circuits might be reduction of additional work needed for manipulation with the links before and after testing i.e. increasing of testing capacity and reduction of cost at high volume testing sites.
During meter testing the source is normally used as a phantom load to provide test currents and voltages applied to both the meters under test and the reference meter. The current flowing into current terminal is supplied separately from the required test voltage. Electronic meter test installations configured in this way allow simultaneously testing of any number of meters limited only by mechanical and power capacity of the test system. The separation of current and voltage measurement circuits at each meter is achieved by disconnecting of links in the terminal block (I-P links). Disconnection avoids interaction of current and voltage circuits and thus large unpredictable measurement error. If the meter under test has closed I-P links, then the interconnection between voltage and current circuit should be eliminated. The principle of testing with closed I-P links is based on isolating the individual voltage and current sources of each meter instead of isolating the voltage and current circuits of the meter. If the test system is dedicated only for Single-phase meters, the isolation of each meter can be realized either using individual isolated voltage source or using individual isolated current source for each installed meter.
The voltage source isolation can be solved either using individual precision voltage transformer advantageously with 1:1 ratio at each meter or using special common voltage transformer with separate output voltage winding for each meter. In this case the test rack must be equipped with multiple voltage wiring network for individual connection of voltage terminals of each meter to respective winding of the common transformer (the transformer is known generally as a multi-secondary voltage isolation voltage transformer or MSIVT). The number of secondary windings is at least equal to the number of meters under test, plus an additional one for the reference meter connection. These transformers are specifically manufactured and calibrated for this purpose and the windings are typically matched to within 0.1%. The additional error introduced by the transformer depends on the load impedance created by the voltage circuit of the tested meters. As the voltage circuit impedance of electricity meter is not subject of metrological specification, the error of the individual winding of the isolation transformer is unpredictable especially at testing different meters.
The required isolation can be achieved by using transformers in the current circuits with one current transformer per phase for each test position. In this way, each meter under test is supplied with isolated test currents. These transformers advantageously have a current ratio of 1:1 and should have amplitude and phase errors over the required current range small enough as not to introduce significant additional errors. The current transformers should be constructed to transfer relatively large (approximately 5 decades) range of current used for meter testing. The size, weight and the cost and of such transformer is serious due to calculation using maximum transferred power and negligible phase error comparable to used precision reference meter.
In classic approach of the non-linearity inherent in these transformers, degrade the overall accuracy of the system at lower currents.
Therefore, it is an object for this invention to provide new and improved arrangement of test circuit with isolating arrangement reducing the size and cost with high precision capability.